The present invention relates to advanced composite materials of the resin-based type impregnated with continuous filament fibers for structural applications, and more particularly to a method of treatment of such materials in a tape-like form to improve their compression strength.
In the field of advanced composite materials, designallowable static strength values for a given composite material are generally utilized to insure the design of sound structural components. Such design-allowable static strengths are statistically determined on the basis of load tests conducted on a number of specimens of a particular composite material and are indicative of load values at which there is a low probability of material failure. Accordingly, higher design-allowable static strengths are preferred in order to provide the utmost efficiency and reliability in the design and construction of composite material structure.
The values of design-allowable static strengths for a particular composite material are dependent upon both the average static strength of the tested specimens and the extent of scatter or variation of the individual test values. Increases in the average static strength of the composite specimens generally raise the design-allowable static strengths for that particular material, while high coefficients of variation in the load test data, indicative of a large scatter of strength properties, result in low design-allowable values. Composite materials typically demonstrate rather large coefficiants of variations which limit their design-allowable values, and such large variation coefficients are particularly evident in those composites of the resin-base type impregnated with fiber columns for structural reinforcement of the resin base. Such impregnated material, commercially available in a tape-like ply that is layered and cured to fabricate a laminated structure, contain misaligned and distorted fiber columns which adversely affect compression strength of the composite specimens in an irregular manner so that a large range of test results, some significantly below expected performance, is produced.
While various processing methods have been investigated and developed for the precured treatment of such fiber-impregnated, resin-based materials, none have been entirely satisfactory in overcoming the problem of the misaligned and distorted fiber columns that reduce the design-allowable static strength values of the material. In particular, the tape-like form of such composite materials has been susceptible to substantial separation, called "brooming", of the individual tows of the fiber-impregnated material during previous attempts to straighten the fiber columns.